Gallium nitride based semiconductor materials are attractive for electronic and optoelectronic devices. Gallium nitride epitaxial layers and device structures grown on native GaN substrates contain several orders of magnitude fewer threading defects than similar structures grown heteroepitaxially on non-native substrates. However, GaN grown by molecular beam epitaxy on native substrates contains high concentrations (typically >1019 cm−3) of oxygen, carbon, and silicon.
N-polar GaN substrates are often utilized with little or no chemical cleaning prior to placement within the molecular beam epitaxy (MBE) reactor chamber. Homoepitaxial GaN layers grown on such untreated surfaces show evidence of high concentrations of unwanted impurities (i.e. oxygen, carbon, silicon). The existing best practice (see Reference 1) is to employ an ex situ chemical clean using solvents and hydrofluoric acid prior to loading the substrate wafer into the MBE reactor, and then to use several cycles of gallium deposition at low temperature (such as approximately 650° C.) followed by desorption at high temperature (such as approximately 815° C.). This procedure removes oxygen on the surface by converting it to volatile gallium oxide which then desorbs from the surface. This technique has been reported on both GaN (Reference 2) and SiC (Reference 3) substrates.
The reactive surfaces of GaN reduce the effectiveness of surface cleaning procedures performed ex situ (outside the vacuum growth chamber), thus an in situ procedure is desirable in order to reduce the concentrations of these impurities.